1. Field of the Invention
The present invention relates to a flux-cored wire for stainless steel arc welding used for arc welding of duplex stainless steel and the like with austenitic phase and ferritic phase, and relates specifically to a flux-cored wire for stainless steel arc welding excellent in the pitting corrosion resistance and the low temperature toughness as well as excellent in weldability of welding in all attitudes.
2. Description of the Related Art
With respect to a member for a structure of chemical plant equipment, oil well pipe for excavation of petroleum or natural gas, line pipe, chemical tanker, water gate and the like, excellent stress corrosion crack (SCC) resistance and pitting corrosion resistance as well as high strength in an environment including mainly chlorine ion of sea water and the like have been conventionally required. For a member satisfying these requirements, duplex stainless steel of JIS SUS329J3L, SUS 329J4L and ASTM S31803 and the like, for example, have been used.
For welding a member of these structures, welding material with an alloy composition similar to that of a base metal is commonly used basically. Accordingly, similar to the base metal, alloy composition of the welding material is diversified. Further, various kinds of welding methods are employed, and arc welding using highly efficient and easily usable flux-cored wire is widely employed in particular.
However, the duplex stainless steel is subjected to heat treatment after rolling in the manufacturing process, and the structure of the duplex stainless steel is stabilized to a structure close to an equilibrium state at the heat treatment temperature. On the other hand, the weld metal is of comparatively unstable structure of a non-equilibrium state in which an austenitic phase is precipitated in a ferritic phase in a natural cooling step after solidification in a single ferritic phase. Accordingly, in the weld bead of the duplex stainless steel, a plane unstable in the pitting corrosion resistance or the low temperature toughness when compared with the steel exists, deterioration of the pitting corrosion resistance and/or the low temperature toughness becomes a problem in some structures employing the same, and therefore improvement of the welding material is desired for.
As the technology for improving the pitting corrosion resistance of the weld bead in arc welding using a flux-cored wire, U.S. Published Patent Application No. 2008/0093352 and Japanese Published Unexamined Patent Application No. 2008-221292 can be exemplarily cited. These patent documents disclose the technology for improving the pitting corrosion resistance of the weld bead by adding Cr, Mo and N to the flux-cored wire. Also, it is disclosed that the strength of the weld metal is improved by adding N to the flux-cored wire.
However, the conventional technologies described above include the following problems. In recent years, because the structural material such as the duplex stainless steel has come to be used in a high pressure and low temperature environment including the chlorine ion of the submarine trench, seabed and the like for example, higher pitting corrosion resistance and low temperature toughness have been required for the weld bead. However, the conventional flux-cored wire described above has not coped with the requirements in recent years on the pitting corrosion resistance and the low temperature toughness.
Also, with regard to the flux-cored wire described in U.S. Published Patent Application No. 2008/0093352, the content of the alkaline metal composition is low, the arc stability cannot be secured and the weldability deteriorates particularly in vertical upward welding.
Further, with regard to the flux-cored wire for welding duplex stainless steel described in Japanese Published Unexamined Patent Application No. 2008-221292, the ZrO2 content is low and excellent slag encapsulating performance cannot be secured particularly in vertical welding and overhead welding, thereby the slag removability and the porosity resistance deteriorate, and the weldability deteriorates.